SF6 gas traces were injected into Moulin L41A, and samples were collected in the proglacial river near Leverett Glacier terminus (67.09N, -50.23E). A total of 11 traces were carried out between 16 June and 6 August, 2012. Tracers were injected at ~19:00 hrs, shortly after the peak daily moulin discharge (typically between 15:00 and 19:00 hrs). Tracer concentrations were monitored by collecting water samples from the proglacial river draining Leverett Glacier and were analysed by a Cambridge Scientific 300-series gas chromatograph fitted with an electron capture detector. Analytical errors (estimated from the analysis of duplicate samples) were below 15% and the level of detection of SF6 was below 0.005 parts per trillion by mass in water. The analytical method was more sensitive than that employed in our previous study at this site (Chandler et al., 2013, http://dx.doi.org/10.1038/ngeo1737) and is described in detail by Chandler et el. (In review, Earth and Planetary Science Letters)

Tracer injection times are local time (UTC-3). Times after injection are the time in hours after the gas was released into the moulin. SF6 concentration is the concentration in the proglacial river, reported as parts per trillion by mass in water, and is usually the mean of duplicate samples.